Oncogene database

1869

E2F1

E2F transcription factor 1;E2F1;E2F transcription factor 1

PBR3|PRB-binding protein E2F-1|RBAP-1|RBBP-3|retinoblastoma-associated protein 1|retinoblastoma-binding protein 3|transcription factor E2F1

20q11.2

protein-coding

Using a transgenic mouse model expressing the E2F1 gene under the control of a keratin 5 (K5) promoter, we previously demonstrated that increased E2F1 activity can promote tumorigenesis by cooperating with either a v-Ha-ras transgene to induce benign skin papillomas or p53 deficiency to induce spontaneous skin carcinomas. We now report that as K5 E2F1 transgenic mice age, they are predisposed to develop spontaneous tumors in a variety of K5-expressing tissues, including the skin, vagina, forestomach, and odontogenic epithelium. On the other hand, K5 E2F1 transgenic mice are found to be resistant to skin tumor development following a two-stage carcinogenesis protocol. Additional experiments suggest that this tumor-suppressive effect of E2F1 occurs at the promotion stage and may involve the induction of apoptosis. These findings demonstrate that increased E2F1 activity can either promote or inhibit tumorigenesis, dependent upon the experimental context.

Loss of retinoblastoma (Rb) tumor suppressor function, as occurs in many cancers, leads to uncontrolled proliferation, an increased propensity to undergo apoptosis, and tumorigenesis. Rb negatively regulates multiple E2F transcription factors, but the role of the different E2F family members in manifesting the cellular response to Rb inactivation is unclear. To study the effect of deregulated E2F4 activity on cell growth control and tumorigenesis, transgenic mouse lines expressing the E2F4 gene under the control of a keratin 5 (K5) promoter were developed, and their phenotypes were compared to those of previously generated K5 E2F1 transgenic mice. In contrast to what has been observed in vitro, ectopically expressed E2F4 was found to localize to the nucleus and induce proliferation to an extent similar to that induced by E2F1 in transgenic tissue. Unlike E2F1, E2F4 does not induce apoptosis, and this correlates with the differential abilities of these two E2F species to stimulate p19(ARF) expression in vivo. To examine the role of E2F4 in tumor development, the mouse skin two-stage carcinogenesis model was utilized. Unlike E2F1 transgenic mice, E2F4 transgenic mice developed skin tumors with a decreased latency and increased incidence compared to those characteristics in wild-type controls. These findings demonstrate that while the effects of E2F1 and E2F4 on cell proliferation in vivo are similar, their apoptotic and oncogenic properties are quite different.

The ARF tumour suppressor protein (p14(ARF) in human and p19(ARF) in mouse) is a major mediator of the activation of p53 in response to oncogenic stress. Little is known about the signalling pathways connecting oncogenic stimuli to the activation of ARF. Regulation of ARF occurs primarily at the transcriptional level and several modulators of ARF transcription have been identified. Notably, ectopic expression of E2F1 upregulates ARF transcriptionally, and both E2F1 and ARF have been implicated in apoptosis and cell-cycle arrest. We have used primary mouse fibroblasts deficient for E2F1, E2F2, or both to determine the possible role of these E2F proteins as upstream regulators of ARF in response to oncogenic stimuli and other stresses. In particular, we have studied the effects of oncogenic Ras and the viral oncoprotein E1A on ARF levels, neoplastic transformation, and sensitization to apoptosis. We have also examined the behaviour of the E2F-deficient MEFs with respect to immortalization and sensitivity to DNA damage. None of the ARF-mediated responses that we have analysed is significantly affected in E2F1(-/-), E2F2(-/-) or E2F1/2(-/-) MEFs, and ARF is upregulated normally in ALL cases. Taken together, our results indicate that the activation of ARF in response to oncogenic stress can occur by E2F1- and E2F2-independent mechanisms. This challenges previous suggestions implicating E2F factors as key mediators in the activation of ARF by oncogenic stress.

cancer arises as a result of a stepwise accumulation of genetic changes. One of these changes, deregulation of the Rb/E2F1 pathway resulting from alterations in members of the pathway, is a hallmark of ALL human cancers. These mutations promote tumor development by deregulating the E2F family of transcription factors, which results in uncontrolled cell cycle progression. The E2F1 protein functions as a transcription factor that enhances cell proliferation by binding to the promoter region of several genes, including those that are involved in cell cycle regulatory activities and DNA replication. It is now becoming clear that the role of E2F1 in regulating transcription and cell growth is also highly dependent on the cellular context. This complexity is also evident from analyses of perturbations in E2F-modulated tumor development. For example, deregulated E2F1 expression can either promote or inhibit tumorigenesis depending on the nature of the other oncogenic mutations that are present. This explains the ability of E2F1 to behave as both an oncogene and tumor suppressor gene. Here we focus on reviewing the most recent evidence supporting the "addiction" of gliomas to this versatile transcription factor. We also consider the clinical relevance of this by examining the role of E2F1 as a prognosis factor and as a target for the development of novel strategies.

Current treatments of sporadic Burkitt s lymphoma (sBL) are associated with severe toxicities. A better understanding of sBL formation would facilitate development of less toxic therapies. The etiology of sBL remains, however, largely unknown, C-MYC up-regulation being the only lesion known to occur in ALL sBL cases. Several studies examining the role of C-MYC in the pathogenesis of BL have concluded that C-MYC translocation is not the only critical event and that additional unidentified factors are expected to be involved in the formation of this tumor. We herein report that a gene distinct from C-MYC, E2F1, is involved in the formation of ALL or most sBL tumors. We found that E2F1 is highly expressed in Burkitt s lymphoma cell lines and sBL lymphoma specimens. Our data indicate that its elevated expression is not merely the consequence of the presence of more cycling cells in this tumor relative to other cell lines or to other neoplasias. In fact, we show that reduction of its expression in sBL cells inhibits tumor formation and decreases their proliferation rate. We also provide data suggesting that E2F1 collaborates with C-MYC in sBL formation. E2F1 expression down-regulation did not affect, however, the proliferation of human primary diploid fibroblasts. Because E2F1 is not needed for cell proliferation of normal cells, our results reveal E2F1 as a promising therapeutic target for sBL.

The analysis of DNA tumor viruses has provided landmark insights into the molecular pathogenesis of cancer. A paradigm for this field has been the study of the adenoviral E1a protein, which has led to the identification of proteins such as p300, p400, and members of the retinoblastoma family. Through binding Rb family members, E1a causes deregulation of E2F proteins--an event common to most human cancers and a central pathway in which oncogenes, including E1a, sensitize cells to chemotherapy-induced programmed cell death. We report here, however, that E1a not only causes deregulation of E2F, but importantly that it also causes the posttranscriptional upregulation of E2F1 protein levels. This effect is distinct from the deregulation of E2F1, however, as mutants of E2F1 impaired in pRb binding are induced by E1a and E2F1 induction can also be observed in Rb-null cells. Analysis of E1a mutants selectively deficient in cellular protein binding revealed that induction of E2F1 is instead intrinsically linked to p400. Mutants unable to bind p400, despite being able to deregulate E2F1, do not increase E2F1 protein levels and they do not sensitize cells to apoptotic death. These mutants can, however, be complemented by either the knockdown of p400, resulting in the restoration of the ability to induce E2F1, or by the overexpression of E2F1, with both events reenabling sensitization to chemotherapy-induced death. Due to the frequent deregulation of E2F1 in human cancer, these studies reveal potentially important insights into E2F1-mediated chemotherapeutic responses that may aid the development of novel targeted therapies for malignant disease.

PURPOSE: To elucidate new molecular mechanisms able to downregulate the mRNA levels of key oncogenes, such as B-Myb and E2F1, in a therapeutic perspective. EXPERIMENTAL DESIGN: B-Myb and E2F1 mRNA levels were evaluated in primary B chronic lymphocytic leukemia (B-CLL, n = 10) and acute myeloid leukemia (AML, n = 5) patient cells, in a variety of p53(wild-type) and p53(mutated/deleted) leukemic cell lines, as well as in primary endothelial cells and fibroblasts. Knockdown experiments with siRNA for p53 and E2F1 and overexpression experiments with miR34a were conducted to elucidate the role of these pathways in promoting B-Myb downregulation. RESULTS: In vitro exposure to Nutlin-3, a nongenotoxic activator of p53, variably downregulated the expression of B-Myb in primary leukemic cells and in p53(wild-type) myeloid (OCI, MOLM) and lymphoblastoid (SKW6.4, EHEB) but not in p53(mutated) (NB4, BJAB, MAVER) or p53(deleted) (HL-60) leukemic cell lines. The transcriptional repression of B-Myb was also observed in primary normal endothelial cells and fibroblasts. B-Myb downregulation played a critical role in the cell-cycle block in G(1) phase induced by Nutlin-3, as shown by transfection experiments with specific siRNA. Moreover, we have provided experimental evidence suggesting that miR-34a is a central mediator in the repression of B-Myb both directly and through E2F1. CONCLUSIONS: Owing to the role of B-Myb and E2F1 transcription factors in controlling cell-cycle progression of leukemic cells, the downregulation of these oncogenes by miR-34a suggests the usefulness of therapeutic approaches aimed to modulate the levels of miR-34a.

One of the defining features of aggressive melanomas is their complexity. Hundreds of mutations and an ever increasing list of changes in the transcriptome and proteome distinguish normal from malignant melanocytic cells. Yet, despite this altered genetic background, a long-known attribute of melanomas is a relatively low rate of mutations in the p53 gene. However, it is unclear whether p53 is maintained in melanoma cells because it is required for their survival, or because it is functionally disabled. More pressing from a translational perspective, is to define whether there is a tumor cell-selective wiring of p53 that offers a window for therapeutic intervention. Here, we provide genetic and pharmacological evidence demonstrating that p53 represents a liability to melanoma cells, which they thwart by assuming an oncogenic dependency on the E3 ligase murine double minute-2 (MDM2). Specifically, we used a combination of RNA interference and two structurally independent small molecule inhibitors of the p53-MDM2 interaction to assess the relative requirement of both proteins for the viability of normal melanocytes and a broad panel of melanoma cell lines. We demonstrated in vitro and in vivo that MDM2 is selectively required to blunt latent pro-senescence signals in melanoma cells. Notably, the outcome of MDM2 inactivation depends not only on the mutational status of p53, but also on its ability to signal to the transcription factor E2F1. These data support MDM2 as a drug target in melanoma cells, and identify E2F1 as a biomarker to consider when stratifying putative candidates for clinical studies of p53-MDM2 inhibitors.

Previous studies have revealed that the link between c-Myc and E2F1 pathway plays a pivotal role in regulating cell growth and death. Human telomerase reverse transcriptase (hTERT), activation of which is required for cell immortalization and transformation, has been confirmed to be a direct transcriptional target of c-Myc. It is of note that E2F1, which is also a direct transcriptional target of c-Myc, can bind the hTERT promoter and repress its expression. Thus, although oncogene c-Myc can be activated in normal cells, for the subsequent induction of E2F1, it may still be insufficient to trigger the expression of hTERT. This negative feedback regulation, if it exists, may be another mechanism for normal cells to control the transmission of c-Myc-mediated oncogenic signals. In this article, we reviewed current knowledge about the crosstalk among c-Myc, E2F1 and hTERT, with an emphasis on the hypothesis that E2F1 negatively regulates c-Myc-induced hTERT transcription. Additionally, we postulated that the miR-17-92 cluster-mediated regulation of c-Myc and E2F1 expression may be of particular importance for the repression of hTERT transcription.

E2F-1, a key transcription factor necessary for cell growth, DNA repair, and differentiation, is an attractive target for development of anticancer drugs in tumors that are E2F "oncogene addicted". We identified a peptide isolated from phage clones that bound tightly to the E2F-1 promoter consensus sequence. The peptide was coupled to penetratin to enhance cellular uptake. Modeling of the penetratin-peptide (PEP) binding to the DNA E2F-1 promoter demonstrated favorable interactions that also involved the participation of most of the penetratin sequence. The penetratin-peptide (PEP) demonstrated potent in vitro cytotoxic effects against a range of cancer cell lines, particularly against Burkitt lymphoma cells and small cell lung cancer (SCLC) cells. Further studies in the H-69 SCLC cell line showed that the PEP inhibited transcription of E2F-1 and also several important E2F-regulated enzymes involved in DNA synthesis, namely, thymidylate synthase, thymidine kinase, and ribonucleotide reductase. As the PEP was found to be relatively unstable in serum, it was encapsulated in PEGylated liposomes for in vivo studies. Treatment of mice bearing the human small cell lung carcinoma H-69 with the PEP encapsulated in PEGylated liposomes (PL-PEP) caused tumor regression without significant toxicity. The liposome encapsulated PEP has promise as an antitumor agent, alone or in combination with inhibitors of DNA synthesis.

NELL2 was first identified as a mammalian homolog of the chicken NEL protein. It was expressed in neurons and has been suggested to play a role in cell survival. However, no clear evidence has yet been available for functions of NELL2. In this study, we found two E2F1 binding sites located in the NELL2 promoter region. We examined the expression of NELL2 and E2F1 in human breast cancer cells (MDA-MB231, MCF7) and bladder cancer cells (5637, UC5). In MDA-MB231 and 5637, the expression levels of NELL2 and E2F1 were higher. To examine the interaction between E2F1 and NELL2, the binding activity was checked by a promoter assay and chromatin immunoprecipitation. From the results, we suggest that NELL2 is a novel target gene of E2F1, which is a key regulator of cell proliferation. We reveal that expression of NELL2 is regulated by E2F1, specifically, mRNA and protein levels of NELL2 are elevated upon activation of exogenous E2F1. Moreover, cells overexpressing NELL2 increased their invasive ability and an enhancement of the effect was observed when NELL2 and E2F1 were coexpressed in MDA-MB231 cells. Therefore, we suggest a novel activity for NELL2 in cancer progression through the regulation of E2F1.

cancerous inhibitor of protein phosphatase 2A (CIP2A) is a recently identified oncoprotein that is overexpressed in many human malignant tumors including cervical cancer. Human papillomavirus (HPV) oncoprotein E7 is the key transformation factor in cervical cancer. Our previous data showed a positive association of CIP2A and HPV-16E7 protein levels; however, how CIP2A is regulated by HPV-E7 and the roles of CIP2A in HPV-E7-mediated cell proliferation are unknown. In this study, we demonstrated that HPV-16E7 protein significantly upregulating CIP2A mRNA and protein expression depended on retinoblastoma protein pRb rather than p130. CIP2A siRNA knockdown in HPV-E7-expressing cells inhibited cell proliferation, DNA synthesis and G1/S cell cycle progression. CIP2A siRNA decreased the protein levels of cyclin-dependent kinase 1 (Cdk1), Cdk2 and their partner cyclin A2, with no change in levels of Cdk4, Cdk6 and their partner cyclin D1. The downregulation of Cdk1 and Cdk2 was independent of c-Myc; instead, E2F1 was the main target of CIP2A in this process, as overexpression of E2F1 rescued the inhibitory effects of CIP2A siRNA knockdown on cell proliferation and G1 arrest of HPV-E7-expressing cells. Our studies reveal a novel function of CIP2A in HPV-16E7-mediated cell proliferation.

The MDM2 proto-oncogene is found amplified in a variety of tumours. The oncogenic capacity of the MDM2 protein is attributed to its ability to bind the p53 tumour-suppressor protein and mask its transcriptional activation potential. Here we show that MDM2 makes a functional contact with two cooperating transcription factors, E2F1 and DP1 (refs 4,5), which are involved in S-phase progression. MDM2 contacts the activation domain of E2F1 using residues conserved in the activation domain of p53. However, in contrast to its repression of p53 activity, MDM2 stimulates the activation capacity of E2F1/DP1. These results indicate that MDM2 not only releases a proliferative block by silencing the tumour suppressor p53, it also positively augments proliferation by stimulating the S-phase inducing transcription factors E2F1/DP1.